1. Technical Field
The present invention relates to an oscillator in which an oscillation frequency is subjected to temperature compensation by using a variable capacitance element. Further, the invention relates to an electronic device, a moving object, and the like, that use such an oscillator.
2. Related Art
For example, in a temperature compensated crystal oscillator (TCXO), a variable capacitance element is used to perform temperature compensation for an oscillation frequency. A TCXO in the related art has a circuit configuration in which gain of an oscillation stage circuit is constant regardless of the temperature, in order to maintain constant amplitude of an oscillation signal. However, although the gain of the oscillation stage circuit is theoretically constant, the oscillation signal does not have the constant amplitude in a case where a crystal resonator has temperature-dependent impedance. When the amplitude of the oscillation signal changes, the absolute value and linearity of the sensitivity of the variable capacitance element changes.
In addition, the variable capacitance element has temperature-dependent sensitivity, and the absolute value and the linearity of the sensitivity of the variable capacitance element changes depending on the temperature. In this respect, when the temperature changes, the impedance of the crystal resonator or the sensitivity of the variable capacitance element changes and thus, the oscillation frequency of the TCXO changes. However, in recent years, there has been a high demand for high accuracy, and the TCXO is required to maintain a constant oscillation frequency even though the temperature changes.
As related technology, Japanese Patent No. 4985035 (paragraphs [0014] to [0018] and FIG. 1) discloses, in FIG. 1, an oscillation circuit that has an oscillation frequency with high accuracy even in a case where the temperature characteristics are significantly different for each sample. The oscillation circuit includes a reference resistor RE1 that generates a reference current, an integrated circuit 2 that is provided separately from the reference resistor, that has an operational amplitude AMP1 which supplies a current to the reference resistor, a reference voltage generating circuit 24 that determines a reference voltage which is applied to the reference resistor, and a constant-voltage circuit 21 which generates a constant voltage, and that determines the oscillation frequency based on the reference current and the constant voltage, and a register 25 that sets the temperature dependence of the reference voltage which is output from the reference voltage generating circuit such that the temperature dependence is the same as the temperature dependence of the reference resistor.
According to the oscillation circuit disclosed in Japanese Patent No. 4985035 (paragraphs [0014] to [0018] and FIG. 1), the reference resistor is provided outside the integrated circuit and thus, it is possible to use a resistor element having a small temperature dependence, compared to a case where the reference resistor is provided inside the integrated circuit. In addition, a value is set, for each reference resistor, to the resistor that sets the temperature dependence of the reference voltage, and thereby, even in a case where the temperature dependence of the reference resistor is different for each sample, it is possible to set the temperature dependence of the reference voltage in response to the difference. In this manner, even in a case where the temperature dependence of the resistor element is significantly different for each sample, it is possible to generate a constant current that is not dependent on the temperature.
However, it is complicated to set, in the register, the temperature dependence of the reference voltage for each sample. In addition, although a constant current that is not dependent on the temperature is supplied to the oscillation stage circuit of the TCXO, the impedance of the crystal resonator or the sensitivity of the variable capacitance element is dependent on the temperature and thus, it is not possible to maintain a constant oscillation frequency of the TCXO.